The circuitry of most AIMDs is susceptible to the magnetic fields generated by magnetic resonance imaging (MRI) machines or other devices that generate magnetic fields similar to MRI machines. Thus, certain patients with AIMDs are not capable of undergoing an MRI procedure. Without proper shielding, such magnetic fields would interfere and possibly render the circuitry in the AIMD's inoperable.
There are a number of patents that discuss compatibility with MRI machines and the need for compatibility of AIMDs with magnetic fields such as those generated by MRI machines. These patents cover a wide variety of topics, including the need to protect implanted lead wires, as well as the need to protect AIMDs, such as cardiac pacemakers. U.S. Pat. No. 5,217,010, assigned to the Johns Hopkins University, describes a number of embodiments regarding electrically shielded housings. FIG. 17 of U.S. Pat. No. 5,217,010 illustrates such a housing, which consists of composite layers 302, 304, and describes the shield assembly as being a continuous non-magnetic metal case that prevents currents from being induced inside the internal pacemaker circuitry. All present titanium housings perform this function. The shielded housing disclosed in U.S. Pat. No. 5,217,010 is either a single layer or consists of laminates as shown in FIG. 17 where the laminated housing contains alternating metal and insulating layers. U.S. Pat. No. 5,217,010 describes FIG. 17 having metal layers 302, 306 and 310 and insulating layers 304, 308 and 312. According to U.S. Pat. No. 5,217,010, this embodiment reduces heating and other interference with proper pacemaker function caused by current flowing between pacemakers/sensing electrodes and the case of the pacemaker. The laminated housing divides up the pre-existing titanium case into separated layers thereby reducing current losses in the presence of a strong magnetic field. The primary objective of the structure in U.S. Pat. No. 5,217,010 is to reduce heating of the housing during exposure to magnetic fields. Some studies have indicated that the amount of heating is not generally an issue with present MRI technology. Accordingly, some degree of heating is acceptable, even desirable, in order to prevent the MRI field energy from reaching sensitive circuits within the AIMD. It is desirable not to have the shielded housing of the AIMD heat up more than about 2° C.-3° C. during an MRI procedure. A temperature rise of more than 3° C. could become quite uncomfortable for the patient and may cause damage to adjacent tissues.
Accordingly, a methodology is desired to apply a magnetic shield coating of various densities and various magnetic and material properties and to control its thickness such that only an acceptable amount of heating is permitted, but not so much heating as to cause discomfort or damage to patient tissue. There are a number of other patents that describe magnetic shielding, shielded conductors or housings that include: U.S. Pat. Nos. 6,506,972; 5,540,959; 6,673,999; 6,713,671; 6,760,628; 6,765,144; 6,815,609; 6,829,509; and 6,901,290.
U.S. Pat. No. 6,506,972 describes magnetically shielded conductor assemblies covered with a nanomagnetic material as described in the patent. There is nothing in U.S. Pat. No. 6,506,972 that describes the coating or shielding of the housing of an active implantable medical device. U.S. Pat. No. 5,540,959 describes a process for preparing a coated substrate in which a mist of particles is created. U.S. Pat. No. 6,673,999 is a Continuation-in-Part of U.S. Pat. No. 6,506,972, which is directed toward the coating and protection of leads and related assemblies.
U.S. Pat. No. 6,713,671 describes a shielded assembly containing a substrate and a shield. It primarily describes a magnetic shielding coating. As shown in FIG. 1a of U.S. Pat. No. 6,713,671, there is a nanomagnetic material coating, a heat treatment and then a coating of insulator material. In FIG. 29 a composite shield assembly that shields from magnetic and/or electric fields is shown. A number of materials are also described. In column 28, line 35 and column 30, line 55, U.S. Pat. No. 6,713,671 describes the various features. The description of the shield 3004 is that it is “disposed above the substrate 3002. As used herein, the term ‘above’ refers to a shield that is disposed between a source 3006 of electromagnetic radiation and the substrate 3002. The shield 3004 is comprised of from about 1 to about 99 weight percent of nanomagnetic material 3008; such nanomagnetic material, and its properties are described elsewhere in this specification.” Col. 28, line 65-Col. 29, line 4. Column 29, lines 9-17, states “[r]eferring again to FIG. 29, and in the preferred embodiment depicted therein, it will be seen that the shield 3004 is also comprised of another material 3010 that preferably has an electrical resistivity of from about 1 microhm-centimeter to about 1×1025 microhm-centimeters. This material 3010 is preferably present in the shield at a concentration of from about 1 to about 99 weight percent, and more preferably, from about 40 to about 60 weight percent.” The patent goes on to further describe said material 3010 as a carbon nanotube material.
U.S. Pat. No. 6,760,628 is primarily directed to a shielded fiber optic system that is addressed to MRI. U.S. Pat. No. 6,765,144 describes an assembly for shielding implantable medical devices from the effects of high frequency radiation and from MRI signals. The assembly includes an implanted medical device and a magnetic shield composed of nanomagnetic material disposed between the medical device and the high frequency radiation. U.S. Pat. No. 6,765,144 describes FIGS. 24, 25 and 26 as depicting a layered magnetic shield using various nano-materials. However, U.S. Pat. No. 6,765,144 does not disclose a continuous metallic electromagnetic shield as part of its assembly. U.S. Pat. No. 6,815,609 is very similar to U.S. Pat. No. 6,765,144, in that a magnetically shielded substrate assembly includes a substrate and a magnetic shield disposed over the substrate. The above comments pertaining to U.S. Pat. No. 6,765,144 also apply to U.S. Pat. No. 6,815,609.
U.S. Pat. No. 6,829,509 discloses an electromagnetic immune tissue invasive system which is primarily a fiber optic system with some description of electrically shielded electrical lead system. None of the features of U.S. Pat. No. 6,829,509 are practical in the context of the present invention.
U.S. Pat. No. 6,901,290 discloses an electromagnetic immune tissue invasive system that includes control circuits contained within a primary housing having an electromagnetic shield. The shield disclosed in U.S. Pat. No. 6,901,290 is a metallic sheath, a carbon composite sheath, or a polymer composite sheath the purpose of which is to shield the primary device housing and any circuits therein from electromagnetic interference. Alternatively, the lead system may comprise a plurality of electrical leads, each lead having a similar shield therearound to prevent the electrical leads from conducting stray electromagnetic interference. In addition to the shield or in lieu of the shield, each electrical lead may include an electrical filter that comprises capacitive and inductive filter elements adapted to filter out predetermined frequencies of electromagnetic interference. In either embodiment, the shield has a biocompatible surface such as a non-permeable diffusion resistant biocompatible material. The shield can be formed of various composite materials so as to provide an electromagnetic shield around the primary housing. Examples of such materials are metallic shielding or polymer or carbon composites such as carbon fullerenes.
Accordingly, an AIMD with improved magnetic and electrical shielding is needed that is simpler in design and construction so as to require less space and expense while properly shielding internal circuitry from electric and magnetic fields and permitting the intentional actuation of a reed switch, Hall-effect device, embedded telemetry coil or other magnetically actuated device within such AIMD. The disclosed invention fulfills these needs and provides other related advantages.